Thermometer for remote temperature measurements

ABSTRACT

An instrument is provided for measuring the temperature of an object from infrared radiation emitted by the object. The instrument includes a radiation detector, a temperature indicating device connected to the detector, and a modulator disposed in a path of a radiation beam from the object for converting the beam into a series of pulses when the modulator is vibrated in and out of the path. The modulator includes a primary piezoelectric element adapted to vibrate when subjected to driving signals at a frequency related to the resonant frequency of the modulator, and a secondary piezoelectric element connected with and driven by the primary piezoelectric element. The secondary piezoelectric element is electrically insulated from the primary piezoelectric element. The modulator also includes a blocking element connected with the secondary piezoelectric element and disposed to move in and out of the path of the beam. A phase locked loop oscillator circuit is connected to the primary and secondary piezoelectric elements for operating the primary element. The secondary piezoelectric element is adapted to generate a signal indicative of the phase and frequency of the primary piezoelectric element to lock the circuit at the resonant frequency of the modulator.

This application is a division of application Ser. No. 08/149,864, whichwas filed on Nov. 10, 1993, now U.S. Pat. No. 5,391,001.

FIELD OF THE INVENTION

This invention relates generally to temperature measuring devices and,more particularly, is directed towards an improved device for measuringthe temperature of a remote object, including a novel resonant circuit.

BACKGROUND OF THE INVENTION

U.S. Pat. No. 3,925,668 discloses a temperature measuring devicedesigned to remotely measure the temperature of an object, i.e., thetemperature is measured without direct contact with the object. Thedevice is useful in various clinical and industrial applications and forvarious consumer uses. Radiant energy from the object being monitored issensed by the device by use of an aperture, optical filter,electro-mechanical modulator and a radiation sensor along withappropriate electronic circuitry and a temperature indicator outputdisplay. The modulator for the foregoing device consists of a magnet, amoving coil and a vibrating needle disposed in the path of theradiation, which is adapted to change the radiation impinging upon thesensor from a steady state to a pulsed state.

U.S. Pat. No. 4,233,512 also discloses a thermometer for making remotetemperature measurements, including a temperature sensor element and amodulator for converting radiation emitted by the object being monitoredfrom a steady state to a pulsed state. The modulator consists of apiezoelectric ceramic reed element adapted to chop the radiation at aprecise, fixed frequency prior to impingement on the sensor element. Thepiezoelectric device forms part of a resonant circuit in which thedevice serves as an active element in a phase locked loop arrangementused to drive and stabilize the modulator. The piezoelectric devicedeflects under applied voltage and also generates a signal that is usedto lock the circuit onto the resonant frequency of the device.

One object of the present invention is to provide a remote temperaturemeasuring device having a modulator with improved means for generating afeed back signal for phase lock control of the modulator. Another objectof the invention is to extend the frequency range under which themodulator can be operated to provide useful resonant motion.

SUMMARY OF THE INVENTION

The present invention is directed to an improved modulator for aninstrument for measuring the temperature of an object from infraredradiation emitted by the object. The instrument includes a radiationdetector adapted to generate an electrical output in response toradiation impinging thereon, a temperature indicating device connectedto the detector for converting the electrical output of the detectorinto a display representative of the temperature of the object, and amodulator that is disposed in a path of the radiation beam forconverting the beam into a series of pulses when the modulator isvibrated in and out of the path. The modulator includes a primarypiezoelectric element adapted to vibrate when subjected to drivingsignals at a frequency related to the resonant frequency of themodulator, and a secondary piezoelectric element connected with anddriven by the primary piezoelectric element. The secondary piezoelectricelement is electrically insulated from the primary piezoelectricelement. The modulator also includes a blocking element connected withthe secondary piezoelectric element and disposed to move in and out ofthe path of the beam. A phase locked loop oscillator circuit isconnected to the primary and secondary piezoelectric elements andprovides driving signals to the primary element. The secondarypiezoelectric element is adapted to generate a reference signalindicative of the phase and frequency of the primary piezoelectricelement to lock the circuit at the resonant frequency of the modulator.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a perspective view of an instrument for making remotetemperature measurements.

FIG. 2 is a schematic diagram of the primary operating components of aprior art remote temperature measuring device.

FIGS. 3-5 are perspective views schematically illustrating the behaviorof a piezoelectric element in response to an applied DC voltage.

FIG. 6 is a chart illustrating the wave forms in the FIG. 2 circuit atdifferent points thereof.

FIG. 7 is a perspective view of another prior art device.

FIG. 8 is a side elevation view of modulating device of an instrumentfor making remote temperature measurements in accordance with oneembodiment of the invention.

FIG. 9 is a top plan view of the device shown in FIG. 8.

FIG. 10 is a chart illustrating the relative ranges of motion of thevarious elements of the device shown in FIGS. 8 and 9.

FIG. 11 is a perspective view of a modulating device in accordance withan alternative embodiment of the invention.

FIG. 12 is a perspective view of a portion of the device shown in FIG.11, illustrating its sideways chopping motion.

FIG. 13 is a schematic diagram of a control circuit according to thepresent invention for driving the modulating device shown in FIG. 11.

FIG. 14 is a schematic diagram of an alternative circuit according tothe present invention for driving the modulating device shown in FIG.11.

DETAILED DESCRIPTION

FIGS. 1-7 illustrate a thermometer with its primary components and theoperation thereof as disclosed in U.S. Pat. No. 4,233,512. FIG. 1 showsa small, light weight, portable thermometer 10 adapted to remotely readthe temperature of an object (not shown). The thermometer 10 may beused, for example, as a clinical thermometer for taking the temperatureof a patient without making physical contact with the patient. Thethermometer 10 comprises a housing 12 including an aperture 14 at oneend thereof that may be directed toward the patient or object whosetemperature is to be measured. If the subject is a patient, the deviceis typically directed towards his or her mouth or, alternately, someother part of the body where a temperature abnormality is suspected. Atthe opposite end of the housing 12 in the top wall thereof, a gauge 16is provided with a movable needle 18 that moves across the gauge face toindicate the temperature measured by the instrument. Other types ofreadout devices like digital displays may be utilized in place of thegauge shown.

FIG. 2 illustrates in schematic form the primary operating components ofa prior art temperature measuring instrument. As shown, infrared energyfrom a patient, etc., passes through the aperture 14 as a steady beam 20and through a filter 22 before passing through a modulator or chopper24, which converts the steady beam 20 into a pulsed beam 26 prior toimpingement upon an infrared detector 28. The detector 28 converts thepulsed IR energy 26 into pulsed electrical energy, which is amplified at30 and processed through analog circuits to drive the display meter 16.The conversion of the steady state IR energy to a pulsed state avoids DCdrift that might otherwise occur, thereby increasing the accuracy of theinstrument. The IR sensor 28 may be one of various devices adapted toconvert IR energy to electrical energy. Use of thin lead sulfide devicesas the IR sensor has provided satisfactory results.

The modulator 24 is a piezoelectric ceramic device in the form of a reed32 that physically deflects when a DC voltage is applied thereto. FIGS.3-5 illustrate the reaction of the piezoelectric reed when a DC voltageis applied to it. In FIG. 3, the device deflects or is bent upwardlywhen a positive voltage is applied to the upper half of the reed and anegative voltage is applied to its lower half. In FIG. 4, the reed isshown at rest when no voltage is applied to it, and in FIG. 5, the reedis bent downwardly when a negative voltage is applied to the upper halfof the reed and a positive voltage applied to the lower half. In FIGS.3-5, X indicates the displacement of the device due to bending actionthereof when subjected to the voltage, P indicates the direction of thepolarization, V indicates the voltage applied, and L indicates thelength of the piezoelectric reed.

The piezoelectric ceramic reed 32 may be dimensioned such that thereed's natural resonant frequency of one quarter wave length or amultiple of one quarter wave length falls into an audio frequency rangeof 40 to 600 Hz. The piezoelectric ceramic material is classified as a"motor" or "bender" implying that a mechanical motion is produced as aresult of applying an electric potential. These materials will alsoproduce the reverse effect by generating a voltage as a result ofmechanical deformation, although the choice of a single material may notbe optimal for both functions. If a pulsed voltage is applied to thereed, it will deflect in the manner shown in FIGS. 3-5 and vibrate at asteady amplitude. Chemically, these materials are identified astitinates or zirconates of lead or barium.

FIG. 2 illustrates a circuit adapted to drive the reed 32 at itsresonant point and hold it at that point under normal operatingconditions as typically might be found if the circuit were incorporatedin a medical temperature sensing instrument. The illustrated circuit hasthe inherent ability to not only search out the resonant frequency ofthe reed, but to also lock itself on that frequency once it isestablished.

In the FIG. 2 circuit, the piezoelectric reed element 32 forms anintegral, active part of the circuit in addition to providing chopperfunctions for the IR energy directed against the sensor 28. The circuitincludes a voltage controlled oscillator ("VCO") 34 that provides thedriving signals to the piezoelectric reed 32 and that also provides anerror signal to a phase comparator 36. A low pass filter 38 receives theoutput of the phase comparator and provides an input to the VCO 34. Anamplifier and low pass filter 40 serves to amplify the reference outputsignal of the reed 32 and return it to the phase comparator signal input36. The circuit is adapted to supply its own reference frequency as wellas a phase shifted error signal. These two signals are applied to thephase comparator.

The normal, free running frequency of the VCO 34 is set approximatelyclose to the fundamental resonant frequency of the piezoelectric reed32. This is a square wave with a 50% duty cycle at the output of the VCOat point A. The same wave form appears on both sides of a capacitor C₁.This capacitor is electrically in series with the reed 32, which itselfappears as a capacitor due to its metal-ceramic-metal sandwichconfiguration. Both capacitors in conjunction with a resistor R₁ act asa differentiating network for the applied square wave resulting in apositive and negative spike appearing at point B. These spikes representthe driving impulses that cause the piezoelectric reed to bendphysically in a near sinusoidal manner and create a usable displacementat the free end of the reed to provide the optical chopping of the beam20. Point B, however, indicates that there is an additional waveformpresent that occurs between the positive and negative spikes and, atresonance, this waveform approximates a sine wave connecting to thespikes on the positive and negative excursions. The source of thissecond waveform is the piezoelectric reed 32 generating its own outputvoltage as a result of its bending. This generator effect produces a lowvoltage measurable output and is not masked by the driving signal,because the driving signal is AC coupled to the piezoelectric anddifferentiated at point B. These waveforms are illustrated in FIG. 6.

The output signal generated by the piezoelectric reed 32, afteramplification at 40, is phase compared with its driving signal in aphase locked loop configuration. Employing a phase comparator thatoperates in quadrature, 90° between its two inputs, and applying thepiezo generator signal to its input, the piezoelectric reed 32 will bedriven towards its resonant frequency until phase lock occurs. If thecenter frequency of the voltage controlled oscillator is set near theresonant frequency of the reed, it will shift in response to the phaseerror voltage produced by the low pass loop filter. Phase dynamics ofthe system will drive the reed to resonance whether the free runningfrequency of the VCO is above or below the resonant frequency.

The self seeking and locking features make this piezo element andcircuit very useful as a radiation modulator or other device operatingin the audio frequency range where a stable locking state is rapidlyachieved. The circuit has a minimum number of components and tolerancesmay be low. In addition, resonance stability is achieved withouttemperature compensation, expensive crystals, mixing or dividingcircuits.

FIG. 7 illustrates an alternative piezoelectric reed design. Instead ofthe reed 32 being disposed directly along the optical path of the beam20 as shown in FIG. 2, a reed 32' is mounted on a fixed support 42 atone end and at the opposite free end carries a stiff, lightweight,optically dense shield 44. The shield 44 is attached to the free end ofthe reed 32' and extends across the optical path of the beam 20' tointerrupt the beam and chop it prior to impingement against a radiationsensor 28'. When the reed 32" is subjected to a pulsed signal, it willvibrate in the manner described above and the shield 44 will oscillateacross the optical path to chop the beam 20' The offset configurationprovided by the shield on the reed allows more flexible and convenientarrangements for the components of the optical system. The addition ofthe shield will add some mass to the reed, which will result in someshift of its resonant frequency in a slightly downward direction.

The values of the resistor and capacitor in the circuit must be lowenough to differentiate the driving signal from the self generatedsignals of the piezoelectric reed. Typically C₁ may have a value of 0.1to 0.2 MFD. The amplifier 40 should have low pass characteristics inorder to eliminate spikes from passing on to the comparator 36. Thecomparator 36 should see only the signals generated by the piezoelement, otherwise the spikes, if passed, will make the circuit unstableand prevent the desired phase locking action from occurring. Thus, it isimportant to separate the driving signals clearly from those signalsgenerated by the piezoelectric reed. The function of the capacitor C₁ isto provide DC isolation for the piezoelectric reed itself.

FIGS. 8 and 9 are side and top views, respectively, of an iinstrumentmodulator 124 for an instrument for remotely measuring temperature inaccordance with the invention. The modulator 124 comprises a primarypiezoelectric element 150, a secondary piezoelectric element 152, and ablocking element 154. The primary piezoelectric element 150 is mountedon a fixed support 142 at one end thereof similar to the piezoelectricelements 32 and 32' of FIGS. 2 and 7, respectively. The blocking element154 is a thin, flexible metallic part that is generally "L" shaped,comprising a horizontal member 156 and a vertical member 158. Thevertical member 158 serves as an optical interrupter like the element 32of FIG. 2 or the shield 44 of FIG. 7; it converts a steady beam 120 ofradiation energy from the object (not shown) whose temperature is to bemeasured into a pulsed beam 126 prior to impingement upon an infrareddetector 128. The detector 128, like the detectors of FIGS. 2 and 7,converts the pulsed IR energy into electrical energy. The detector 128may be connected with appropriate temperature indicating means (similarto that shown in FIG. 2) that convert the electrical output of thedetector to a display representative of the temperature of the object.

The end of the horizontal member 156 of the blocking element 154 distalto the vertical member 158 is attached to one end of the secondarypiezoelectric element 152. The other end of the secondary piezoelectricelement 152 is attached to the primary piezoelectric element 150. Theresonant frequency of the modulator 124 may be adjusted by varying thelengths of the piezoelectric elements 150, 152 and the blocking element154.

Like the prior art piezoelectric elements 32, 32' of FIGS. 2 and 7,respectively, the primary piezoelectric element 150 alone provides onlylimited mechanical motion at frequencies exceeding 100 Hz. This isprimarily due to the material stiffness of the element and its length toresonant frequency relationship. The modulator 124 in accordance withthe present invention, however, provides useful resonant motion at anextended frequency range by adding the thin, flexible blocking element154 to the piezoelectric elements 150, 152. The frequency range isextended such that useful resonant motion can occur at frequencies ashigh as 1 KHz. FIG. 10 illustrates the ranges of motion of the primaryand secondary piezoelectric elements 150, 152 and the blocking element154 of the modulator 124 when the primary piezoelectric element 150 isvibrated by driving signals supplied thereto. As shown, the range ofmotion of the primary piezoelectric element 150 is significantlyamplified by the blocking element 154.

Like the prior art modulators shown in FIG. 2 and 7, the modulator 124is connected with control circuitry (not shown) adapted to drive themodulator 124 at its resonant point and hold it at that point undernormal operating conditions. However, the resonant circuit formed by themodulator 124 of the present invention differs from the prior art asdescribed below.

The prior art modulators of FIGS. 2 and 7 each comprise a singlepiezoelectric vibrating element 32, 32' that is both energized bydriving signals to vibrate and that generates a control or feedbacksignal for phase lock control. However, detection of the generatedfeedback signal requires very low level signal separation from the muchlarger driving signal, leading to start-up and signal captureuncertainties. The present invention overcomes this problem by providingtwo piezoelectric elements: the primary piezoelectric element 150, whichis energized by driving signals from the circuit, and the secondarypiezoelectric element 152, which is mechanically driven by the primaryelement 150 and which generates the feedback control signal. Thesecondary piezoelectric element 152 is mechanically joined to theprimary piezoelectric element 150, but is electrically insulated fromthe primary element 150. Because the secondary piezoelectric element 152comprises a piezoelectric device, it will generate an electrical signalin response to and indicative of its driven motion. Since the secondaryelement 152 is also electrically isolated from the primary element 150,the electrical signal output provided by the secondary element 152 willdescribe the phase and frequency of the primary element 150 withoutrequiring use of processing or amplification devices for its detection.

The modulator 124 forms an integral, active part of the control circuit,which also includes a voltage controlled oscillator and a phasecomparator. The voltage controlled oscillator provides driving signalsto the primary piezoelectric element 150 and also provides an errorsignal to the phase comparator. The secondary piezoelectric element 152provides the signal indicative of the phase and frequency of the primarypiezoelectric element 150 to the phase comparator. The phase comparatorprovides an output that is a function of the signals received from theoscillator and the secondary piezoelectric element to the oscillator.The circuit is thus adapted to drive the modulator to its resonant pointand to hold it at that point during operation of the temperaturemeasuring instrument.

FIG. 11 illustrates a modulator 224 in accordance with anotherembodiment of the invention. The modulator 224 comprises a primarypiezoelectric element 250, a secondary piezoelectric element 252 and athin, flexible metallic blocking element 254. The primary element 250has a first end fixed to a modulator base 242 and an opposite second endcoupled mechanically, but not electrically, to one end of the secondarypiezoelectric element 252. An insulating element 251 is positionedbetween the piezoelectric elements 250, 252. The other end of thesecondary piezoelectric element 252 is secured to the blocking element254. The blocking element 254 is generally "L" shaped, comprising ahorizontal member 256 and a vertical member 258.

Like the modulator 124 of FIGS. 8 and 9, the primary piezoelectricelement 250 is energized by driving signals from a control circuit, andthe secondary piezoelectric element 252 is mechanically driven by theprimary element 250 and generates a feedback control signal in responseto its driven motion. As illustrated in FIG. 13, the modulator 224 formsan integral, active part of the control circuit, which also includes avoltage controlled oscillator 270, a phase comparator 272, and a lowpass filter 274. The voltage controlled oscillator 270 provides drivingsignals to the primary piezoelectric element 250 and also provides anerror signal to the phase comparator 272. The secondary piezoelectricelement 252 provides the signal indicative of the phase and frequency ofthe primary piezoelectric element 250 to the phase comparator 272. Thephase comparator 272 provides an output that is a function of thesignals received from the oscillator 270 and the secondary piezoelectricelement 252 to the oscillator 270. The circuit is thus adapted to drivethe modulator to its resonant point and to hold it at that point duringoperation of the temperature measuring instrument.

The vibration of the primary piezoelectric element 250 causes thevertical blocking member 258 to move in a sideways chopping motion toconvert a steady radiation beam 220 from the object (not shown) whosetemperature is being measured into a pulsed IR beam 226. The pulsed beam226 is received by the detector 228, which converts the pulsed IR energyinto electrical energy. The detector 228 is connected with appropriatetemperature indicating means that convert the electrical output of thedetector to a display representative of the temperature of the object.

FIG. 14 illustrates a self-oscillating multi-vibrator circuit as analternative to the phase locked loop drive circuit of FIG. 13. Like theFIG. 13 circuit, the FIG. 14 circuit can be used to drive the modulatorsshown in FIGS. 8 or 11. The FIG. 14 circuit requires fewer componentsand less power, making it attractive for less demanding portableapplications.

In the FIG. 14 circuit, the piezoelectric elements 250, 252 areanalogous to a quartz crystal controlling the frequency of anoscillator, where mechanical resonance replaces electrical resonance. Inthe circuit, the piezo elements 250, 252 are depicted as capacitorsbecause of their similar construction and electrical charge storingproperties.

In operation, powering up of the circuit causes shock excitation of theprimary piezo element 250 and sets into motion the secondary piezoelement 252. The output of the secondary element 252 through a phasecompensation network 360 is a positive feedback voltage which initiatesoscillation in the amplifier 362. As the sustained oscillation ischanging frequency and amplitude, the circuit will seek the resonantpoint of the piezo element pair 250, 252 and lock at that value. Theoscillation frequency will remain under tight control of the resonantcharacteristics of the piezo elements 250, 252 because the feedbacksignal is derived from and is part of the frequency determining elementin the loop. A maximum deflection of the interrupter 258 occurs for agiven circuit supply voltage. This circuit conveniently yields anelectrical output signal at point A (FIG. 14) that can provide areference signal for driving a "synchronous detection system."

While circuits formed by the modulators disclosed herein have beendisclosed with particular reference to their use in conjunction withtemperature measuring instruments, their phase locking characteristicsmake them attractive for use in other circuits where a stable modulatingcircuit is desired. The piezoelectric element is, however, particularlyuseful in the thermometer instrument of the type disclosed in view ofits low power requirements, stability, simplicity and low cost. It makesa very practical battery-operated, hand-held temperature measuringinstrument, providing quick, reliable performance. In practice, aperson's temperature may be measured almost immediately since theresponse time of the instrument is on the order of a few milliseconds.

The present invention has been described in the foregoing specificationwith respect to specific embodiments that serve as examples toillustrate the invention rather than to limit its scope. Modificationsmay be made to the described embodiments without departing from thebroader teachings of the invention.

What is claimed is:
 1. A modulator for an instrument for measuring thetemperature of an object from infrared radiation emitted by the object,the instrument including a radiation detector adapted to generate anelectrical output in response to radiation impinging thereon, a devicefor forming and directing a beam of radiation against the detector, atemperature indicator connected to the detector to convert theelectrical output of the detector into a display representative of thetemperature of the object, and a circuit for operating the modulator,the modulator being disposed in a path of the radiation beam andvibrated in and out of the path to convert the beam into a series ofpulses, the modulator having a resonant frequency and comprising:aprimary piezoelectric element adapted to vibrate when subjected todriving signals from the circuit at a frequency related to the resonantfrequency of the modulator; a secondary piezoelectric element connectedwith said primary piezoelectric element to vibrate with said primaryelement, said secondary piezoelectric element being electricallyinsulated from said primary piezoelectric element, and said secondarypiezoelectric element generates and provides a reference signalindicative of the frequency and phase of the primary piezoelectricelement to the circuit to lock the circuit at the resonant frequency ofsaid modulator.
 2. The modulator of claim 1, further comprising ablocking element secured to said secondary piezoelectric element andwherein said blocking element is disposed in the path of said beam andmoves in and out of said path when the primary piezoelectric element isvibrated.
 3. The modulator of claim 2 wherein said blocking element isgenerally "L" shaped, comprising a horizontal member and a verticalmember.